This invention relates to wireless communications devices, and more particularly to lithium-ion battery chargers for wireless communications devices.
Conventional battery chargers for wireless communications devices typically charge a battery using a two-step charging process. Initially, a constant current is supplied to the battery. Once the battery voltage increases to a predetermined level, the charging current into the battery is permitted to vary while the battery voltage is held constant. The battery charger includes charge control components that typically vary a controlled impedance in series with the battery to control the charging of the battery.
A problem with conventional battery chargers is that the controlled impedance may dissipate an excessive amount of power during the early stages of constant current charging, depending on the discharge state of the battery. A discharged battery has a relatively low terminal voltage compared to a fully charged battery. Therefore, during the early stages of constant current charging a higher voltage is impressed across the controlled impedance, resulting in increased dissipation. The increased dissipation leads to reduced reliability and possible failure of the controlled impedance.
A method of charging a battery from a power source is provided. The method includes providing a charge circuit in series with the battery and the power source. A charging current flowing to the battery and a voltage across the charge circuit are sensed. The power dissipated in the charge circuit is computed based on the charging current and the voltage across the charge circuit. During a first operating mode, the charging current is controlled so that the power dissipated in the charge circuit is about a predetermined maximum dissipation. During a second operating mode, the charging current is limited to a predetermined current level. During a third operating mode, the charging current is controlled so that the battery voltage is about a predetermined voltage level.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.